224 Proceedings of the Royal Irish Academy. 



handsomely developed crystals of sphene. There are signs of defor- 

 mation in some of the felspars ; but the gneissic structure is clearly 

 due to other than dynamic causes. As in previous cases, the epidote 

 is in constant association with the streaks of biotite or hornblende. 



It is noteworthy that the rounded granules of pyrite observed in 

 the pure euritic aplite of this locality occur also in the more aplitic 

 layers of the composite rocks. 



There is good evidence, then, that the varieties of grey and red 

 gneiss above Finntown are portions of granite masses locally modi- 

 fied by the conditions of their intrusion. The phenomena of Ballyir- 

 iston and Derkbeg Hill are here carried out on a still more convincing 

 scale. It seems highly probable that the "Hornblende-Biotite-Granites 

 (Hornblende-Granitites) " described from other parts of Donegal by 

 Dr. Hyland^ have the same origin as those studied in the present 

 paper. 



What, then, is the general conclusion that we may come to in 

 regard to the gneissic patches, often half a mile or so long, which 

 occur with so constant a foliation- strike in the granite mass south of 

 the Gweebarra ? Are they not, equally with the strips of epidiorite 

 and limestone, the relics of strata that formerly occurred, in metamor- 

 pbic wrinklings, in the crown of some great anticlinal arch^ ? It is 

 not necessary to urge that the whole of the space now occupied by 

 granite was formerly filled with folded schists, and that the solution 

 of the latter provided room for the granite in its ascent ; the magma 

 of the granite may have at first welled up into the spaces provided for 

 it by the Caledonian folding, and then, under continued earth -pressures, 

 have been forced, with destructive effect, against its bounding roof 

 and walls. 



Instead of representing, as Mr. Scott was tempted to do, the 

 foliated and sedimentary strips of rock near Finntown as vertical 

 beds between vertical layers of granite, may we not rather regard the 

 present surface of the mass between Lough Finn and Doocharry 

 Eridge as exhibiting a cross-section of the upper zone of inter- 

 action? Some masses from the roof have survived, and give us a 

 profound impression of the material that has been altogether lost 

 within the granite caldron (fig. 1). Had denudation worn away the 



1 Mem. to Sheets 3, 4, 15, etc., p. 13G. 



- Mr. Kilroe speaks of detached areas and bands of schist, etc., in the granite 

 east of Dimgloe, which form a curved series when regarded as a whole, as 

 " obviously fragments of a schist series, which formerly extended from Tor 

 westward" {Ibid., p. 43). 



